The invention relates to a fluid clutch, composed of a primary part with a drive disc and a secondary part with a cover. Included are primary and secondary parts mounted to be rotatable with respect to one another, wherein the drive disc is located in a working chamber formed in the secondary part. The working chamber communicates via a return channel with a supply chamber, likewise located in the secondary part, for returning the clutch fluid, whereby the return of the clutch fluid takes place from the space left between an axial circumferential surface of the drive disc and a radial bordering surface of the working chamber. Retarders for increasing the return pressure as well as a control element are provided thereby making possible the return, depending on the rotational direction, of the drive disc.
Such fluid clutches, also known as viscosity clutches, are known in many embodiments. The torque between the clutch parts is transmitted with zero wear by a highly viscous fluid without the clutch parts coming in contact. Depending on the quantity of the clutch fluid, which provides the torque transmission between the drive disc and the working chamber, the clutch action of the primary and secondary parts takes place with different transmission factors. The quantity of clutch fluid in the working chamber, hereinafter referred to as the working fluid, is controlled in known designs in such a way that, for example, a valve bore is provided in one wall, which divides the interior of the clutch into a working chamber and a supply chamber. Depending on the operating conditions, this valve bore is open to a greater or lesser extent, thus permitting the clutch fluid in the supply chamber to be transferred to the working chamber. In order to be able to bring the working fluid from the working chamber back into the supply chamber, known methods provide retarders which retard the working fluid in the vicinity of the circumference of the drive disc, and return it to the supply chamber via a return channel. These retarders can be mounted either on the cover or on the drive disc. In any case, corresponding recesses must be provided either in the cover or on the drive disc, made, for example, in the form of annular grooves, in order to achieve the desired effect.
In addition to retarders, which act in only one rotary direction, other types are known which are suitable for application in clutches which can operate in both rotational directions. A system of this kind, which is suitable for returning the working fluid from both rotary directions of the drive disc relative to the secondary part is known from German Offenlegungsschrift No. 27 50 289. Two elevations on the cover of the secondary part are provided as retarders, which do not extend over the full width of the drive disc in the axial direction and enclose an inner chamber in which an axially directed valve roller is mounted. An opening to the return channel is provided in the middle of this interior chamber. The roller can be displaced in the chamber depending on the direction in which it is influenced in such a way that it acts in one rotational direction or the other either as a retarder or a control element. Its circumference abuts the outer edge of the drive disc as well as the inner edge of the space formed between the elevations. The disadvantage of this arrangement is that the roller is subjected to considerable wear because of the frictional contact. The continuous friction between the outer surface of the drive disc and the roller itself produces a reduction of its diameter, so that, after a certain amount of time, the retarding function of the roller deteriorates. As a result, the required quantity of working fluid is no longer pumped from the working chamber into the supply chamber. The roller, therefore, cannot act reliably as a retarder and control element.